Do I have to buy avast again because my hard disk died?

I purchased avast for my laptop about 6 months ago but my hard disk died on me and now have to replace it.

Do I have to repay to download avast again?

You don’t have to buy it again. Just use this form http://www.avast.com/en-eu/resend-license-paid.php and your license will be resend by mail.